Today, there exist many types of electronic devices such as mobile telephones, media players, GPS modules, palmtop computers, cameras etc. Such devices comprise a user control interface having an output device, for example in the form of a display for presenting visual information such as letters, digits and also graphics and symbols related to the operation of the device in question. For example, such an output device can be a liquid crystal display. The user control interface normally also comprises an input device for manually inputting commands or making selections related to the operation of the device.
An input device for a user control interface can for example be constituted by a touch panel arrangement, which according to prior art can be in the form of a resistive touch panel. Such an arrangement is common today in mobile telephones and similar handheld devices.
According to known technology, a resistive touch panel can be based on a design comprising two conductive layers which are arranged with an air gap between the layers. One of the layers constitutes a top layer upon which the user may press with a finger or a stylus pen at a certain position which corresponds to a predetermined input command. The conductive layers are furthermore arranged so that they come into contact with each other when the user presses upon the top layer. By feeding suitable voltages to the two layers, the position of the input command can be detected.
The touch panel arrangement is normally provided with a thin top layer having printed or painted patterns defining visible letters, digits and other symbols, the purpose of which is to allow the user to make input commands during operation of the control interface.
Furthermore, it is common practice to provide a user control interface with means for illumination of the letters, digits and symbols which are to be used. Such means for illumination is normally arranged as a backlight arrangement. In the field of mobile telephones, for example, backlights are often implemented using light emitting diodes (LEDs) or electroluminiscent (EL) films.
This means that a complete touch panel arrangement with backlight devices can be designed as a multi-layer structure with a first layer in the form of a support layer for carrying one or more light sources for providing the backlight. Such a light source can for example be a blue LED which is encapsulated in epoxy material with fluorescent material so as to emit a white light. The white light is directed through a window in a further layer in the multi-layer structure, wherein the further layer can be in the form of a resistive touch panel, and further toward a top layer being designed as a mask layer with a design defining symbols such as letters, digits and other signs and characters. In this manner, the symbols provided in the top layer can be illuminated by means of the backlight.
In order to provide a suitable colour for the symbols in the top layer, a filter can be positioned between the light emitting diode in the support layer and the symbols provided in the top layer. In this manner, the white light from the light emitting diode can be transformed into a suitable colour.
In today's market for mobile telephones and similar portable and handheld terminals, there is a growing demand for increasing possibilities of attractive and flexible design. For example, there is a demand for providing a user control interface with multi-coloured symbols. As an example related to mobile telephones, it is regarded as attractive for the user if the top layer of a user control terminal is designed with different colours for different symbols. Implementing such a multi-coloured design could be seen as an advantage as regards the design and usefulness of the user control interface.
In order to achieve more than one colour using a single light emitting diode as mentioned, the light emitting diode would have to be arranged to emit all the necessary colours mentioned. A light emitting diode arranged for emitting white light could then be used. If, as an example, a touch panel is to be designed with a first letter being red and a second letter being green, it would be necessary to filter the white light through two filters, i.e. a first filter for filtering the white light to red at the position where the first letter is located, and a second filter for filtering the white light to green at the position where the second letter is located on the panel.
Although this previously known solution is acceptable as regards the design, visual appearance and illumination of the user control interface, it has some drawbacks. Firstly, the fact that two different filters are used in order to filter the white light into two different colours means that the white light will be attenuated to a relatively high degree. This means that, in order to provide an acceptable light intensity, a relatively high energy-consumption of the illumination means would be needed. This is a disadvantage in the field of handheld terminals such as mobile telephones where the available battery operating time is an important factor.
A further disadvantage also relates to the fact that, when filters are used, much of the light intensity is attenuated in the filter. The emitted light intensity from the light emitting diode must therefore be high and in the visible spectrum. This creates a problem with light leakage through various components of the user control interface.
Consequently, it can be summarized that new ways of illuminating user control interfaces are demanded by the market for mobile telephones, for example. However, prior technology for meeting this demand is associated with certain disadvantages, as explained above.